This invention relates to a driving circuit for a brushless motor and, more particularly, to such a circuit exhibiting a simplified construction which produces driving currents that are controlled automatically to provide a high starting torque for the motor yet function to drive the motor with high efficiency and without unnecessary power consumption.
In a typical brushless motor, the rotor is formed of a permanent magnet and the stator is formed of plural phase windings. When the windings are selectively energized, the flux derived therefrom interacts with the flux generated by the permanent magnet rotor, resulting in a force exerted upon the rotor to cause rotation. The driving circuit for a brushless motor generally is arranged such that drive currents flow through successive stator winding phases in sequence.
A drive circuit for a brushless motor of the aforementioned type may include a switching device, such as a transistor, SCR, or the like, connected in series with each stator winding phase. The switching devices are triggered sequentially, thereby permitting drive currents to flow through the corresponding phases in proper sequence. The rate at which such switching devices are triggered should be a function of rotor speed; and position sensing elements may be provided to control the triggering of the switching devices in accordance with the rotary position of the rotor.
During an initial start-up operation of a brushless motor, the starting torque desirably should be a maximum. Since torque is dependent upon the currents flowing through the stator windings, the start-up currents should be relatively high. One manner in which the start-up currents can be increased is merely to increase the magnitude of the drive currents which flow through the switching devices. However, the current saturation level of a typical switching device will limit the maximum current magnitude which can flow through the stator windings. Another manner in which the start-up currents can be increased is to increase the period during which each current flows through its respective stator winding phase. As a simple example, if the brushless motor is formed with a two-pole rotor and three stator winding phases, a higher start-up torque will be produced if a current flows through each phase for a duration greater than 360.degree./3 = 120.degree.. This duration, or current angle, should be increased such that successive stator phase drive currents overlap.
Although greater torque generally is required during a motor start-up operation, once the normal operating speed of the motor has been attained, that is, the speed at which the motor was designed to operate, the torque can be reduced. One problem is that if the successive stator phase drive currents have overlapping portions during the motor start-up operation, these overlapping currents during normal motor operation result in a reduction in the average magnetic flux linkage, whereby the motor exhibits relatively poor efficiency. That is, at normal motor speeds, an unnecessarily high amount of power is required to drive the motor.
Another problem is that if a brushless motor drive circuit is designed to be more efficient at normal motor operating speeds, the sequential drive currents flowing through the stator phases should exhibit no overlapping portions so as to avoid reducing the average magnetic flux linkage. However, this generally means that the start-up torque is relatively low. In the aforementioned simplified example wherein the brushless motor is formed of three stator winding phases, a current angle of 120.degree. through each phase will result in a significant ripple factor in the torque. This ripple factor also is undesired.
Another problem associated with brushless motor drive circuits resides in the waveform of the drive currents which flow through the stator winding phases. The force acting upon adjacent conductors included in a stator winding phase is proportional to the magnitude of the current flowing through such conductors. If the driving current waveform is a rectangular pulse having vertical leading and trailing edges, that is, having relatively short attack and decay times, the rapid change in current at these leading and trailing edges is acccompanied by a corresponding rapid change in the force exerted upon adjacent conductors. This, in turn, causes the conductors in the stator winding phase to vibrate, thereby producing a disturbing sound. In order to avoid this problem of vibrating conductors, various capacitors would have to be connected to the switching devices in order to modify the driving current waveform, that is, to increase the attack and decay time of the current pulses. Such capacitors increase the cost and complexity of the current driving circuit.